12,527 research outputs found
Low energy effective gravitational equations on a Gauss-Bonnet brane
We present effective gravitational equations at low energies in a
-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based
on the geometrical projection approach, and we solve iteratively the bulk
geometry using the gradient expansion scheme. Although the original field
equations are quite complicated due to the presence of the Gauss-Bonnet term,
our final result clearly has the form of the Einstein equations plus correction
terms, which is simple enough to handle. As an application, we consider
homogeneous and isotropic cosmology on the brane. We also comment on the
holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.Comment: 10 pages, v2: minor clarification
`Mass without mass' from thin shells in Gauss-Bonnet gravity
Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity.
There is the well known junction condition for the singular part of the stress
tensor intrinsic to the shell, which we also prove to be well defined. There
are also equations relating the geometry of the shell (jump and average of the
extrinsic curvature as well as the intrinsic curvature) to the non-singular
components of the bulk stress tensor on the sides of the thin shell.
The equations are applied to spherically symmetric thin shells in vacuum. The
shells are part of the vacuum, they carry no energy tensor. We classify these
solutions of `thin shells of nothingness' in the pure Gauss-Bonnet theory.
There are three types of solutions, with one, zero or two asymptotic regions
respectively. The third kind of solution are wormholes. Although vacuum
solutions, they have the appearance of mass in the asymptotic regions. It is
striking that in this theory, exotic matter is not needed in order for
wormholes to exist- they can exist even with no matter.Comment: 13 pages, RevTex, 8 figures. Version 2: includes discussion on the
well-defined thin shell limit. Version 3: typos fixed, a reference added,
accepted for publication in Phys. Rev.
Points of General Relativisitic Shock Wave Interaction are "Regularity Singularities" where Spacetime is Not Locally Flat
We show that the regularity of the gravitational metric tensor in spherically
symmetric spacetimes cannot be lifted from to within the
class of coordinate transformations in a neighborhood of a point of
shock wave interaction in General Relativity, without forcing the determinant
of the metric tensor to vanish at the point of interaction. This is in contrast
to Israel's Theorem which states that such coordinate transformations always
exist in a neighborhood of a point on a smooth single shock surface. The
results thus imply that points of shock wave interaction represent a new kind
of singularity for perfect fluids evolving in spacetime, singularities that
make perfectly good sense physically, that can form from the evolution of
smooth initial data, but at which the spacetime is not locally Minkowskian
under any coordinate transformation. In particular, at such singularities,
delta function sources in the second derivatives of the gravitational metric
tensor exist in all coordinate systems of the atlas, but due to
cancelation, the curvature tensor remains uniformly bounded.Comment: This article has been withdrawn since the main result is wrong due to
an computational error. See arXiv:1506.04081 and arXiv:1409.5060 for a
correction of this error and a proof of the opposite statemen
A low-mass HI companion of NGC 1569?
High-sensitivity maps of the large-scale structure of atomic hydrogen in the
starburst dwarf galaxy NGC 1569 show evidence for an HI cloud with a mass of
7*10**6 M_sun, at a projected distance of 5 kpc from the parent galaxy. This
cloud may be a condensation in a low-column-density HI halo or a companion
galaxy/HI-cloud. NGC 1569 and its companion are connected by a low surface
brightness HI bridge. At the edge of NGC1569, the HI bridge coincides with
H_alpha arcs, also detected in soft X-rays.Comment: 5 pages, 4 figures, 1 tabl
Incompressible fluid inside an astrophysical black hole?
It is argued that under natural hypothesis the Fermions inside a black hole
formed after the collapse of a neutron star could form a non compressible fluid
(well before reaching the Planck scale) leading to some features of integer
Quantum Hall Effect. The relations with black hole entropy are analyzed.
Insights coming from Quantum Hall Effect are used to analyze the coupling with
Einstein equations. Connections with some cosmological scenarios and with
higher dimensional Quantum Hall Effect are shortly pointed out.Comment: 30 pages, 2 figures. Accepted for publication on Physical Review D:
references added, typos corrected, test polishe
Statistical scene generation for polarimetric imaging systems
Little publicly available data exists for polarimetric measurements. When
designing task specific polarimetric systems, the statistical properties of the
task specific data becomes important. Until better polarimetric datasets are
available to deduce statistics from, the statistics must be simulated to test
instrument performance. Most imaged scenes have been shown to follow a power
law power spectral density distribution, for both natural and city scenes.
Furthermore, imaged data appears to follow a power law power spectral
distribution temporally. We are interested in generating image sets which
change over time, and at the same time are correlated between different
components (spectral or polarimetric). In this brief communication, we present
a framework and provide code to generate such data.Comment: 5 pages, 3 figures, 3 matlab function
On the Transfer of Metric Fluctuations when Extra Dimensions Bounce or Stabilize
In this report, we study within the context of general relativity with one
extra dimension compactified either on a circle or an orbifold, how radion
fluctuations interact with metric fluctuations in the three non-compact
directions. The background is non-singular and can either describe an extra
dimension on its way to stabilization, or immediately before and after a series
of non-singular bounces. We find that the metric fluctuations transfer
undisturbed through the bounces or through the transients of the
pre-stabilization epoch. Our background is obtained by considering the effects
of a gas of massless string modes in the context of a consistent 'massless
background' (or low energy effective theory) limit of string theory. We discuss
applications to various approaches to early universe cosmology, including the
ekpyrotic/cyclic universe scenario and string gas cosmology.Comment: V2. Minor Clarifications V3. appendix and 2 figures added, typos
corrected, conclusions unchanged 12 pages, 6 figure
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